UICollectionView - didDeselectItemAtIndexPath not called if cell is selected

The issue is, that the cell is selected, but the UICollectionView does not know anything about it. An extra call for the UICollectionView solves the problem:

[collectionView selectItemAtIndexPath:indexPath animated:NO scrollPosition:UICollectionViewScrollPositionNone]; 

Full code:

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
    UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"Cell" forIndexPath:indexPath];
    cell.selected = YES;
    [collectionView selectItemAtIndexPath:indexPath animated:NO scrollPosition:UICollectionViewScrollPositionNone];
    return cell;
}

I keep the question to help someone who may face the same problem.

I was having the same issue, if you investigate further you willl see that not only the diddiselect is not called, but the touch itself is not perceived by the cell. What solved for me was to allow for async execution of the selection and set the selecItem manually as well. Inside cellForItemAt.

       DispatchQueue.main.async {
            cell.isSelected = true
            collectionView.selectItem(at: indexPath, animated: false, scrollPosition: .init())
        }

Use .init() and false on animated if you don't want to see movements. If you don't wrap this with the async block you will see the UI jumping while scrolling.

In my case, I want to change the background of the button in other words the background of the cell in the collection view:

class CustomCVCell: UICollectionViewCell {

override var isSelected: Bool {
        didSet {
            grayBackgroundViewWithImage.image =
                isSelected ? UIImage(named: "") : UIImage()
        }
    }

In the main class where the collection view is stored create this variable:

class CustomViewController: UIViewController {

///save the indexPath of last selected cell
private var lastSelectedIndexPath: IndexPath? }

In viewDidLoad() set this value to false:

customCollectionView.allowsMultipleSelection = false

Further code in data source. In my case, the first cell should be is selected:

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: CustomCVCell.cellID(),
                                                  for: indexPath) as! CustomCVCell
    
    if indexPath.row == 0 {
        lastSelectedIndexPath = indexPath
        cell.isSelected = true
    }
    
    //update last select state from lastSelectedIndexPath
    cell.isSelected = (lastSelectedIndexPath == indexPath)
    
    return cell
}

Further code in the delegate:

///UICollectionViewDelegate
func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
    guard lastSelectedIndexPath != indexPath else { return }

         if let index = lastSelectedIndexPath {
            let cell = collectionView.cellForItem(at: index) as! CustomCVCell
            cell.isSelected = false
          }

          let cell = collectionView.cellForItem(at: indexPath) as! CustomCVCell
          cell.isSelected = true
    lastSelectedIndexPath = indexPath
}

In my case it was caused by using same logic for

func collectionView(_ collectionView: UICollectionView, shouldHighlightItemAt indexPath: IndexPath) -> Bool

as for

func collectionView(_ collectionView: UICollectionView, shouldSelectItemAt indexPath: IndexPath) -> Bool

And when cell was selected shouldHighlightItemAt was returning false - and this prevented me to deselect that cell.

Hope this will save someone's time :)